+1 there's your problem.

Replace the 6 fuel hoses if they are not translucent. This will take care of the o-rings on the ends of the lines. The one return line that connects to the back of the IP requires 3 sealing washers that are not part of the line, so be sure to get those as well.

The fuel pre-filter has an o-ring. On my car the sealing surface in the well the pre-filter sits in had contamination and so was not making a good seal.

Also the black block on the side of the IP with 3 fuel hoses is the fuel control valve. The 4th connection is on the back side of this valve. It feeds fuel into the IP. If this o-ring is bad it will introduce air and the engine will run poorly at idle.

After you do all that you should be back to smooth idle.

The W210 has a supervised glow plug relay with a digital data connection to the engine management system. If you lose a glow plug it will throw a CEL and set a code.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech